Equivalent for "ref new MainPage()" in C++ Windows Runtime -

let's imagine, have application written on c++/cx. root namespace application namespace someapp{...}. , have ref class mainpage{...} in namespace. also, there ref class app{...}, derived windows::ui::xaml::application class. in method app::onlaunched(...) instantiate instance of mainpage class code

auto somevariable = ref new mainpage();

what equivalent code in raw c++ application(without using c++/cx extension)?
tried replace

comptr<iinspectable> somevariable; auto classname = hstringreference(runtimeclass_someapp_mainpage).get(); roactivateinstance(classname, somevariable.getaddressof());

but call returns "regdb_e_classnotreg class not registered". think, c++/cx compiler replaces code with:

• direct call constructor of mainpage
• attempt find activation factory , after call activateinstance.
• some code

what option right?